Dear Doctor: I own a 1998 Subaru Legacy 2.5-liter four-cylinder engine.

The engine kept overheating and the mechanic finally said the problem was a bad cylinder head or head gasket. He suggested I replace the engine vs. replacing the heads. Can I replace the engine with a 2.2-liter instead of the 2.5? — William

Dear William: The first step is to contact the Subaru dealer. Subarus are prone to head gasket failure. There may be some customer goodwill money available. In no way can you install a 2.2-liter vs. the 2.5-liter engine.

Dear Doctor: My friend purchased a new Honda a couple of years ago. Since day one he has been using premium-unleaded gas. I just purchased a new Accord and I am using regular gas 87 octane. Both owner’s manuals recommend the use of regular 87 octane gas. My friend says he gets better gas mileage using the premium gas and that offsets the price. He also says the premium gas is better for the engine. What are your thoughts? — Rosalie

Dear Rosalie: Under normal driving conditions, without any engine modifications, the use of a higher-octane gas is a waste of money. In fact, under some very cold weather conditions, the use of a higher than recommended gasoline can cause hard starting and hesitation. The best way to get the maximum gas mileage is proper maintenance, conscientious driving habits and the use of full synthetic engine oil.

Dear Doctor: What happens to the car’s computer information memory and diagnostics when the battery is disconnected for six months? Will the car remember the information in the spring? — Joe

Dear Joe: When the battery is disconnected for even a minute, the computer loses some information. On the good side, all the information that is programmed in the computer remains. Some non-written information will be lost, but after a couple of drive cycles the computer is back to where it was before the battery was disconnected. The computers now have the ability to relearn as the vehicle is driven.

Dear Doctor: I own a 2004 Chevy TrailBlazer purchased new. The problem: all four windows get scratched when the windows are opened and closed. GM released a bulletin on Nov. 19, 2004 listing the needed repairs. All four windows were replaced twice. Since the second repair only one window has become scratched. — Deb

Dear Deb: I would say the technician who did the work is where the problem lies. You now have only one window that has become scratched. It’s no secret that there are hundreds of updates and technical bulletins on all makes and models of vehicles. Unfortunately, some of the technicians are not fully trained enough to perform updates correctly.

Dear Doctor: I own a 2002 Toyota Corolla. My problem is the gas mileage drops when the weather gets cold. As soon as the weather warms up, the mileage goes up. I spoke to other Corolla owners and they have the same complaint. — Milton

Dear Milton: This is normal and here are the reasons. First, engines work harder in cold weather. An engine also takes longer to warm up and operate in the closed loop mode. Engine oil thickens, as do other fluids, unless full synthetic oil is used. The gasoline blend mixture is also different in the winter for easier starting. If you were to fill the gas tank and drive only on the highway, then you would find the mileage to be very close to that achieved with warmer temperatures.

Dear Doctor: I own a 1998 Toyota Camry with 75,000 miles. Recently while driving on the highway, as I slowed down or stopped, it would stall. I’d restart immediately and run weeks or months without this problem. My dealer could not find the problem. A few weeks later when the engine started to stall, I kept my foot on the gas pedal and the engine did not stall. I described what happens to my mechanic and he said the car needed a fuel pump. Two fuel pumps later and I’m still having the same problem. What is killing the fuel pumps? — Barry

Dear Barry: Why did your mechanic say it was a fuel pump problem? If I was working on the car, then my first check would be for any trouble fault codes, fuel pressure and computer data for sensor values. Next, I’d look on the Identifix Web site for any history of this problem on the model. We have seen EGR, idle air control motors and coolant sensor problems with these vehicles.

Dear Doctor: I drive a 1983 Cadillac DeVille 4100 V-8 with 134,000 miles. After driving on the highway three miles and coming to a stop, the engine runs very rough and has a high idle speed. The engine never acts up when driving 35 to 40 mph. Any suggestions? — Edwin

Dear Edwin: The old 4100 V-8 was prone to have the EGR valve stick partly open after a drive on the highway. This is the first place to look. You may also be able to retrieve computer data information that could help.

Send questions to: Auto Doctor, 3 Court Circle, Lakeville, MA 02347